Much of the basic research being done today in the rubber industry is directed to defining the content of the rubber compositions which will someday comprise the perfect tire. The litmus of the various formulations developed has been a battery of tests defining the rubber compound's physical properties to determine its fitness for particular applications. One property of interest has been the comparative rolling resistance demonstrated by a tire tread containing a modified rubber compound. This property, which relates to the rubber compound's ability to withstand constant energy input at different frequencies, is of obvious practical importance: reducing the rolling resistance of a tire increases the automotive consumer's gas mileage.
On Oct. 27, 1992, U.S. Pat. No. 5,159,009, entitled "Carbon Blacks Modified with Organosilicon Compounds, Method of Their Production and Their Use in Rubber Mixtures," was issued. This patent (hereinafter referred to as the '009 patent) describes a process for preparing silane modified carbon blacks for subsequent use in reinforcing rubber compounds.
The '009 patent shows a process for bonding organosilicon compounds to carbon black, see '009 patent at col. 1 lns. 51-56, which is to be performed before mixing the reaction product with a rubber mixture, see '009 patent at col. 3 lns. 13-16. Hence, in this disclosure, the '009 process is sometimes termed a "pretreatment" process, because the carbon black is treated in a carefully prescribed manner before being compounded into a rubber mixture.
The '009 pretreatment process comprises: (a) homogeneously mixing one or more of the specified organosilicon compounds with the carbon black in a vessel; (b) heating the components for 1 to 60 minutes at 50.degree. C. to 160.degree. C. in the mixing vessel or another suitable tempering unit; and then (c) extracting any excess unreacted organosilicon compounds with an organic solvent. See '009 patent at col. 2 lns. 29-39.
The particulars of the '009 patent's mixing-heating-extracting process are emphasized throughout the specification. For example, the heating step is termed the "second stage" of the process, see '009 patent at col. 5 lns. 6-11, and the three-stage process shown is uniformly employed in reacting all carbon blacks and silanes used in the examples provided, see '009 patent at col. 5 lns. 12-13.
The extraction step is also emphasized in the patent. Extraction of excess unreacted organosilicon compounds is stated as necessary, because "Extractable amounts of this compound (these compounds) which exceed this amount not only do not result in any further improvement of the properties of the vulcanizate but on the contrary result in a worsening of the rubber technology properties." '009 patent at col. 2 lns. 53-58 (emphasis added). The extraction step could be omitted by using stoichiometric quantities of the organosilicon compounds; this would ensure that there was no excess requiring removal. See '009 patent at col. 2 lns. 59-68; col. 5 lns. 15-44.
The consequences of failure to adhere to the process of the '009 patent is emphasized in two examples in the patent. In Example VI, there is provided a comparison of two natural rubber compounds, both of which employed silane-modified N110 carbon blacks. The first employs an amount of silane which corresponds precisely to the bondable/stoichiometric amount; the second employs an excess of silane. The addition of the excess silane results in a deterioration in (a) tensile strength (lower), (b) modulus 300% (lower), (c) curing time (higher), and (d) delta T center (higher). See '009 patent at col. 8 lns. 13-50.
In Example VII, there is provided a comparison of two SBR (styrene-butadiene rubber) compounds employing N375 carbon black. In the first rubber composition, the carbon black is pretreated with Si69 in accord with the teachings of the invention. The second rubber composition employs a 50:50 mixture of the N375 carbon black and Si69; pretreatment of the carbon black is conspicuously absent. The non-pretreated carbon black rubber composition is shown to display a deteriorated (a) tensile strength (lower), (b) modulus 300% (lower), (c) hardness (lower), (d) dynamic elastic modulus (lower), (e) abrasive wear (higher), and (f) loss angle tan delta (higher), as well as displaying unusual rheometer cure properties. See '009 patent at col. 8 ln. 52-col. 9 ln. 18.
Thus, using the '009 process in the manufacture of rubber compounds requires careful adherence to the claimed "two-step" process: (a) pretreatment (i.e., mixing-heating-extracting) of the carbon black with the prescribed silanes; and then (b) addition of the silane-modified carbon black to the rubber compound in a mixer. In addition, the process is highly sensitive to the amount of silane added to the carbon black; the amount must be stoichiometric. Non-stoichiometric additions of silane result in suboptimal properties in the final rubber product. Left unmet is the need for a process which is not as sensitive to the amount of silane relative to the carbon black and which is faster and more cost-effective than the '009 patent's process.